There are many lighting applications that utilize tubular style lamps to generate light. For many general lighting type applications, the tubular lamps are florescent type lamp products. A florescent lamp typically utilizes mercury vapor and a phosphor coating, inside a glass tube. Current from a ballast drives one or more electrodes, typically at the opposite ends of the tube, to excite the mercury vapor to produce ultraviolet (UV) energy. The UV in turn excites phosphor particles in the coating on the interior surface of the tube to emit a spectrum of visible light. Use of the toxic mercury vapor represents a potential environmental hazard in terms of disposal of the tube lamp and/or in the event of accidental breakage.
Also, the spectral characteristic of the light output may be somewhat undesirable, with regard to color temperature and/or color rendering index. The apparent color of the white light output (corresponding to color temperature) may seem unnatural; and a poor color rendering index (CRI) may result in less than desirable illumination of objects of certain colors. The output spectrum of the florescent tubular lamp product is determined by the output spectrum or spectra of the excited phosphors. Improvement in the characteristics of the phosphors and/or adding phosphors with different output spectra may improve the spectral characteristic of the output of the florescent tube lamp, but these solutions often increase costs. Particularly in less costly products, the white light output still often has a somewhat undesirable color temperature and/or a relatively low color rendering index (CRI), so that the output or illumination of some objects with such light appears unnatural and undesirable to many people. Because of the energy efficiency of the tubular florescent lighting products and the like, however, they have been widely adopted, particularly in commercial settings.
Hence, it would be useful to find an alternative technology that is more environmentally friendly and at least for white light applications would provide a higher quality/more desirable spectral characteristic of white light. Of course, any such alternative technology should be as energy efficient as possible, for example, at least more efficient than existing alternatives such as incandescent lamps.
Recent years have seen a rapid expansion in the performance of solid state lighting devices such as light emitting diodes (LEDs); and with improved performance, there has been an attendant expansion in the variety of applications for such devices. For example, rapid improvements in semiconductors and related manufacturing technologies are driving a trend in the lighting industry toward the use of LEDs or other solid state light sources to produce light for general lighting applications to meet the need for more efficient lighting technologies and to address ever increasing costs of energy along with concerns about global warming due to consumption of fossil fuels to generate energy. LED solutions also are more environmentally friendly than competing technologies, such as florescent lamps, because LED based lighting products do not include any toxic mercury vapor.
The actual solid state light sources, however, produce light of specific limited spectral characteristics. To obtain white light of a desired characteristic and/or other desirable light colors, one approach uses sources that produce light of two or more different colors or principal wavelengths and one or more optical processing elements to combine or mix the light of the various wavelengths to produce the desired characteristic in the output light. However, because the resultant light is a mixture of a number of narrow spectra from the source LEDs, resultant light for example exhibits a relatively low CRI, even when the color or color temperature is of a desired characteristic.
In recent years, techniques have also been developed to shift or enhance the characteristics of light generated by solid state sources using phosphors, including for generating white light using LEDs. Phosphor based techniques for generating white light from LEDs, currently favored by LED manufacturers, include UV or Blue LED pumped phosphors. In addition to traditional phosphors, semiconductor nanophosphors have been used more recently. The phosphor materials may be provided as part of the LED package (on or in close proximity to the actual semiconductor chip), or the phosphor materials may be provided remotely (e.g. on or in association with a macro optical processing element such as a diffuser or reflector outside the LED package). The remote phosphor based solutions have advantages, for example, in that the color characteristics of the fixture output are more repeatable, whereas solutions using sets of different color LEDs and/or lighting fixtures with the phosphors inside the LED packages tend to vary somewhat in light output color from fixture to fixture, due to differences in the light output properties of different sets of LEDs (due to lax manufacturing tolerances of the LEDs).
LED devices also have been suggested as replacements for tubular lamps, such as the tubes commonly used in florescent lighting fixtures. A LED light tube for replacing a florescent tube might include a tubular glass bulb and a pair of end caps for connection through a standard socket to an appropriate power supply. Light emitting diodes disposed inside the tubular glass bulb receive power from the supply through the end caps. The LEDs may be visible light LEDs. It has also been suggested that UV LEDs might be used to pump phosphors or quantum dots coated on an interior surface of a glass bulb, which in the context of a tubular lamp would be on an interior surface of the tubular glass bulb.
Hence, solid state lighting technologies have advanced considerably in recent years, and such advances have encompassed any number of actual LED based tubular lamp products as well as a variety of additional proposals for LED based tubular lamps. However, there is still room for further improvement in the context of tubular solid state lamp products, which for example might be adopted as replacements for conventional florescent lamps or other tubular lamps.
For example, for general lighting applications, it is desirable to provide light outputs of acceptable characteristics (e.g. white light of a desired color rendering index and/or color temperature) in a consistent repeatable manner from one instance of a lamp product to another. Of course, to be commercially competitive with alternative lamp technologies requires an elegant overall solution. For example, the product should be as simple as possible so as to allow relatively low cost manufacturing. Relatively acceptable/pleasing form factors similar to those of well accepted tubular lamps may be desirable. Solid state devices have advantages of relatively high dependability and long life. However, within the desired tubular lamp form factor/configuration or a fixture or system incorporating such a tubular lamp, there are a variety of technical issues relating to use of solid state devices that still must be met, such as efficient electrical drive of the solid state light emitters, efficient processing of the light for the desired output and/or adequate dissipation of the heat that the solid state devices generate.